Implantable neurostimulator-implemented method for managing tachyarrhythmia through vagus nerve stimulation

ABSTRACT

An implantable neurostimulator-implemented method for managing tachyarrhythmias through vagus nerve stimulation is provided. An implantable neurostimulator, including a pulse generator, is configured to deliver electrical therapeutic stimulation in a manner that results in creation and propagation (in both afferent and efferent directions) of action potentials within neuronal fibers of a patient&#39;s cervical vagus nerve. Operating modes of the pulse generator are stored. A maintenance dose of the electrical therapeutic stimulation is delivered to the vagus nerve via the pulse generator to restore cardiac autonomic balance through continuously-cycling, intermittent and periodic electrical pulses. A restorative dose of the electrical therapeutic stimulation is delivered to prevent initiation of or disrupt tachyarrhythmia through periodic electrical pulses delivered at higher intensity than the maintenance dose. The patient&#39;s normative physiology is monitored via a physiological sensor, and upon sensing a condition indicative of tachyarrhythmia, is switched to delivering the restorative dose to the vagus nerve.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 15/706,946, filed Sep. 18, 2017, which is a continuation of U.S. patent application Ser. No. 15/274,919, filed on Sep. 23, 2016, which is a continuation of U.S. patent application Ser. No. 13/673,766, filed Nov. 9, 2012, which are hereby incorporated by reference in their entireties.

BACKGROUND

Congestive heart failure (CHF) and other forms of chronic cardiac dysfunction (CCD) are generally attributed to an autonomic imbalance of the sympathetic and parasympathetic nervous systems that, if left untreated, can lead to cardiac arrhythmogenesis, progressively worsening cardiac function and eventual patient death. CHF is pathologically characterized by an elevated neuroexcitatory state and is accompanied by physiological indications of impaired arterial and cardiopulmonary baroreflex function with reduced vagal activity.

CHF triggers compensatory activations of the sympathoadrenal (sympathetic) nervous system and the renin-angiotensin-aldosterone hormonal system, which initially help to compensate for deteriorating heart pumping function, yet, over time, can promote progressive left ventricular dysfunction and deleterious cardiac remodeling. Patients suffering from CHF are at increased risk of tachyarrhythmias, such as atrial fibrillation (AF), ventricular tachyarrhythmias (ventricular tachycardia (VT) and ventricular fibrillation (VF)), and atrial flutter, particularly when the underlying morbidity is a form of coronary artery disease, cardiomyopathy, mitral valve prolapse, or other valvular heart disease. Sympathoadrenal activation also significantly increases the risk and severity of tachyarrhythmias due to neuronal action of the sympathetic nerve fibers in, on, or around the heart and through the release of epinephrine (adrenaline), which can exacerbate an already-elevated heart rate.

VT originates solely in the lower heart in either the ventricular tissue or Purkinje fibers. During VT, electrical signals within the ventricles begin firing abnormally and cause a rapid rate of ventricular contraction; the rate is so rapid that the heart is unable to fill properly, thereby substantially reducing the forward flow, which results in dramatic reduction in the volume of blood ejected through the aortic valve into the peripheral vascular system. This sudden reduction in blood flow can have immediate deleterious consequences since the brain and other vital organ systems require adequate blood perfusion to maintain their biological integrity. When starved of blood, even for short periods of time, vital organ systems can be damaged. The brain is particularly sensitive to reduced cardiac output. Initially, during low flow conditions, such as during VT, the brain's electrical systems are affected, and patient consciousness may be compromised. If this low or no flow condition persists for minutes, brain tissue damage begins. After six to eight minutes, this damage can become permanent and may ultimately lead to chronic impairment or death, unless the hemodynamic compromise caused by the VT is immediately and definitively corrected. During the VT, action potentials circulating in the ventricles collide and interfere with the normal propagation of action potentials from the sinoatrial (SA) node and the resulting rapid heart rate causes the heart chambers to contract prematurely and without adequately filling, thus preventing proper blood flow and causing potentially lethal hemodynamic compromise.

While other forms of tachycardia, specifically supraventricular (SVT) and sinus tachycardia, are relatively benign unless episodic or prolonged, VT can present life-threatening risk and can degenerate into VF, asystole and sudden cardiac death. Both VT and particularly VF, as well as other forms of potentially life-threatening tachyarrhythmias, must be promptly treated to restore the heart to normal sinus rhythm. The current standard of care for treating VT includes, in order of increasing medical urgency, anti-arrhythmic medications, cardioversion, radiofrequency ablation, and heart surgery. Despite VT existing as an underlying complication of chronic cardiac dysfunction, the primary morbidity, CHF, is typically only managed through the prescription of pharmacological agents and application of dietary and lifestyle changes. As well, in contrast to the customary urgent treatment of VT, the medical measures for managing CHF are merely palliative, not curative. Patients often suffer side effects and other comorbidities, in addition to increased VT risk, due to disease progression.

The standard of care for managing CCD in general continues to evolve. For instance, new therapeutic approaches that employ electrical stimulation of neural structures that directly address the underlying cardiac autonomic nervous system imbalance and dysregulation have been proposed. In one form, controlled stimulation of the cervical vagus nerve beneficially modulates cardiovascular regulatory function. Currently, vagus nerve stimulation (VNS) is only approved for the clinical treatment of drug-refractory epilepsy and depression, although VNS has been proposed as a therapeutic treatment of CHF in general and has been demonstrated in canine studies as efficacious in simulated treatment of AF and heart failure, such as described in Zhang et al., “Therapeutic Effects of Selective Atrioventricular Node Vagal Stimulation in Atrial Fibrillation and Heart Failure,” J. Cardiovasc. Electrophysiol., Vol. pp. 1-6 (Jul. 9, 2012), the disclosure of which is incorporated by reference.

Conventional general therapeutic alteration of cardiac vagal efferent activation through electrical stimulation targets only the efferent nerves of the parasympathetic nervous system, such as described in Sabbah et al., “ Vagus Nerve Stimulation in Experimental Heart Failure,” Heart Fail. Rev., 16:171-178 (2011), the disclosure of which is incorporated by reference. The Sabbah paper discusses canine studies using a vagus nerve stimulation system, manufactured by BioControl Medical Ltd., Yehud, Israel, which includes an electrical pulse generator, right ventricular endocardial sensing lead, and right vagus nerve cuff stimulation lead. The sensing lead enables stimulation of the right vagus nerve in a highly specific manner, which involves closed-loop synchronization of the vagus nerve stimulation pulse to the cardiac cycle. An asymmetric tri-polar nerve cuff electrode is implanted on the right vagus nerve at the mid-cervical position. The electrode provides cathodic induction of action potentials while simultaneously applying asymmetric anodal blocks that lead to preferential activation of vagal efferent fibers. Electrical stimulation of the right cervical vagus nerve is delivered only when heart rate increases beyond a preset threshold. Stimulation is provided at an impulse rate and intensity intended to reduce basal heart rate by ten percent by preferential stimulation of efferent vagus nerve fibers leading to the heart while blocking afferent neural impulses to the brain. Although effective in partially restoring baroreflex sensitivity and, in the canine model, increasing left ventricular ejection fraction and decreasing left ventricular end diastolic and end systolic volumes, the degree of therapeutic effect on parasympathetic activation occurs through incidental recruitment of afferent parasympathetic nerve fibers in the vagus, as well as through recruitment of efferent fibers. Efferent stimulation alone is less effective at restoring autonomic balance than bi-directional stimulation.

Other uses of electrical nerve stimulation for therapeutic treatment of various cardiac and physiological conditions are described. For instance, U.S. Pat. No. 6,600,954, issued Jul. 29, 2003 to Cohen et al. discloses a method and apparatus for selective control of nerve fiber activations. An electrode device is applied to a nerve bundle capable of generating, upon activation, unidirectional action potentials that propagate through both small diameter and large diameter sensory fibers in the nerve bundle, and away from the central nervous system. The device is particularly useful for reducing pain sensations in the legs and arms.

U.S. Pat. No. 6,684,105, issued Jan. 27, 2004 to Cohen et al. discloses an apparatus for treatment of disorders by unidirectional nerve stimulation. An apparatus for treating a specific condition includes a set of one or more electrode devices that are applied to selected sites of the central or peripheral nervous system of the patient. For some applications, a signal is applied to a nerve, such as the vagus nerve, to stimulate efferent fibers and treat motility disorders, or to a portion of the vagus nerve innervating the stomach to produce a sensation of satiety or hunger. For other applications, a signal is applied to the vagus nerve to modulate electrical activity in the brain and rouse a comatose patient, or to treat epilepsy and involuntary movement disorders.

U.S. Pat. No. 7,123,961, issued Oct. 17, 2006 to Kroll et al. discloses stimulation of autonomic nerves. An autonomic nerve is stimulated to affect cardiac function using a stimulation device in electrical communication with the heart by way of three leads suitable for delivering multi-chamber stimulation and shock therapy. For arrhythmia detection, the device utilizes atrial and ventricular sensing circuits to sense cardiac signals to determine whether a rhythm is physiologic or pathologic. The timing intervals between sensed events are classified by comparing them to a predefined rate zone limit and other characteristics to determine the type of remedial therapy needed, which includes bradycardia pacing, anti-tachycardia pacing, cardioversion shocks (synchronized with an R-wave), or defibrillation shocks (delivered asynchronously).

U.S. Pat. No. 7,225,017, issued May 29, 2007 to Shelchuk discloses terminating VT in connection with any stimulation device that is configured or configurable to stimulate nerves, or stimulate and shock a patient's heart. Parasympathetic stimulation is used to augment anti-tachycardia pacing, cardioversion, or defibrillation therapy. To sense atrial or ventricular cardiac signals and provide chamber pacing therapy, particularly on the left side of the patient's heart, the stimulation device is coupled to a lead designed for placement in the coronary sinus or its tributary veins. Cardioversion stimulation is delivered to a parasympathetic pathway upon detecting a ventricular tachycardia. A stimulation pulse is delivered via the lead to one or more electrodes positioned proximate to the parasympathetic pathway according to stimulation pulse parameters based on the probability of reinitiation of an arrhythmia.

U.S. Pat. No. 7,277,761, issued Oct. 2, 2007 to Shelchuk discloses vagal stimulation for improving cardiac function in heart failure patients. An autonomic nerve is stimulated to affect cardiac function using a stimulation device in electrical communication with the heart by way of three leads suitable for delivering multi-chamber endocardial stimulation and shock therapy. Where the stimulation device is intended to operate as an implantable cardioverter-defibrillator (ICD), the device detects the occurrence of an arrhythmia, and applies a therapy to the heart aimed at terminating the detected arrhythmia. Defibrillation shocks are generally of moderate to high energy level, delivered asynchronously, and pertaining exclusively to the treatment of fibrillation.

U.S. Pat. No. 7,295,881, issued Nov. 13, 2007 to Cohen et al. discloses nerve branch-specific action potential activation, inhibition and monitoring. Two preferably unidirectional electrode configurations flank a nerve junction from which a preselected nerve branch issues, proximally and distally to the junction, with respect to the brain. Selective nerve branch stimulation can be used with nerve-branch specific stimulation to achieve selective stimulation of a specific range of fiber diameters, restricted to a preselected nerve branch, including heart rate control, where activating only the vagal B nerve fibers in the heart, and not vagal A nerve fibers that innervate other muscles, can be desirous.

U.S. Pat. No. 7,778,703, issued Aug. 17, 2010 to Gross et al. discloses selective nerve fiber stimulation for treating heart conditions. An electrode device is adapted to be coupled to a vagus nerve of a subject and a control unit drives the electrode device by applying stimulating and inhibiting currents to the vagus nerve, which are capable of respectively inducing action potentials in a therapeutic direction in a first set and a second set of nerve fibers in the vagus nerve and inhibiting action potentials in the therapeutic direction in the second set of nerve fibers only. The nerve fibers in the second set have larger diameters than the nerve fibers in the first set. Typically, the system is configured to treat heart failure or heart arrhythmia, such as atrial fibrillation or tachycardia by slowing or stabilizing the heart rate, or reducing cardiac contractility.

U.S. Pat. No. 7,813,805, issued Oct. 12, 2010 to Farazi and U.S. Pat. No. 7,869,869, issued Jan. 11, 2011 to Farazi both disclose subcardiac threshold vagus nerve stimulation. A vagus nerve stimulator is configured to generate electrical pulses below a cardiac threshold, which are transmitted to a vagus nerve, so as to inhibit or reduce injury resulting from ischemia. For arrhythmia detection, a heart stimulator utilizes atrial and ventricular sensing circuits to sense cardiac signals to determine whether a rhythm is physiologic or pathologic. In low-energy cardioversion, an ICD device typically delivers a cardioversion stimulus synchronously with a QRS complex; thus, avoiding the vulnerable period of the T-wave and avoiding an increased risk of initiation of VF. In general, if anti-tachycardia pacing or cardioversion fails to terminate a tachycardia, then, for example, after a programmed time interval or if the tachycardia accelerates, the ICD device initiates defibrillation therapy.

Finally, U.S. Pat. No. 7,885,709, issued Feb. 8, 2011 to Ben-David discloses nerve stimulation for treating disorders. A control unit drives an electrode device to stimulate the vagus nerve, so as to modify heart rate variability, or to reduce heart rate, by suppressing the adrenergic (sympathetic) system. Typically, the system is configured to treat heart failure or heart arrhythmia, such as AF or tachycardia. In one embodiment, a control unit is configured to drive an electrode device to stimulate the vagus nerve, so as to modify heart rate variability to treat a condition of the subject. Therapeutic effects of reduction in heart rate variability include the narrowing of the heart rate range, thereby eliminating very slow heart rates and very fast heart rates. For this therapeutic application, the control unit is typically configured to reduce low-frequency heart rate variability, and to adjust the level of stimulation applied based on the circadian and activity cycles of the subject. Therapeutic effects also include maximizing the mechanical efficiency of the heart by maintaining relatively constant ventricular filling times and pressures. For example, this therapeutic effect may be beneficial for subjects suffering from atrial fibrillation, in which fluctuations in heart filling times and pressure reduce cardiac efficiency.

Accordingly, a need remains for an approach to therapeutically treating chronic cardiac dysfunction, including CHF, and cardiac arrhythmogenesis, specifically tachycardia, through a form of VNS to improve autonomic balance and cardiovascular regulatory function.

SUMMARY OF THE INVENTION

Excessive sustained activation of the sympathetic nervous system has a deleterious effect on long term cardiac performance and increases the risk of tachyarrhythmias, particularly in a patient with CCD. Bi-directional afferent and efferent neural stimulation through the vagus nerve can beneficially restore autonomic balance and improve long term clinical outcome. The neural stimulation is provided in a low level maintenance dose independent of cardiac cycle. VNS delivery can be provided through an implantable vagus neurostimulator and electrode lead, which begins delivering a restorative dose at a higher level of intensity than the maintenance dose upon sensing a condition indicative of spontaneous tachyarrhythmia. VNS maintenance dose delivery is only continually resumed if, during post-suspension monitoring, tachyarrhythmia is not found to recur.

One embodiment provides an implantable neurostimulator-implemented method for managing tachyarrhythmias through vagus nerve stimulation. An implantable neurostimulator, including a pulse generator, is configured to deliver electrical therapeutic stimulation in a manner that results in creation and propagation (in both afferent and efferent directions) of action potentials within neuronal fibers comprising the cervical vagus nerve of a patient. Operating modes are stored in the pulse generator. A maintenance dose of the electrical therapeutic stimulation is parametrically defined and tuned to restore cardiac autonomic balance through continuously-cycling, intermittent and periodic electrical pulses. A restorative dose of the electrical therapeutic stimulation is parametrically defined and tuned to prevent initiation of or disrupt tachyarrhythmia through periodic electrical pulses delivered at higher intensity, which could be higher output current, higher duty cycle, higher frequency, or longer pulse width, than the maintenance dose. The maintenance dose is therapeutically delivered to the vagus nerve independent of cardiac cycle via a pulse generator included in the implantable eurostimulator through at least a pair of helical electrodes electrically coupled to the pulse generator via a nerve stimulation therapy lead. The patient's normative physiology is monitored via a physiological sensor included in the implantable neurostimulator, and upon sensing a condition indicative of tachyarrhythmia, is switched to delivering the restorative dose to the vagus nerve via the pulse generator through the pair of helical electrodes.

A further embodiment provides an implantable neurostimulator-implemented method for managing tachyarrhythmias through vagus nerve stimulation. A maintenance dose of electrical therapeutic stimulation for delivery via an implantable neurostimulator is defined and tuned to restore cardiac autonomic balance through continuously-cycling, intermittent and periodic electrical pulses. A restorative dose of electrical therapeutic stimulation for delivery via the implantable neurostimulator is defined and tuned to prevent initiation of or disrupt tachyarrhythmia through periodic electrical pulses delivered at higher intensity, which could be higher output current, higher duty cycle, higher frequency, or longer pulse width, than the maintenance dose. Vagus nerve stimulation therapy is provided to the vagus nerve through at least a pair of helical electrodes via a stimulation therapy lead electrically coupled to a pulse generator included in the implantable neurostimulator. The maintenance dose is therapeutically delivered via the pulse generator to the vagus nerve independent of cardiac cycle in a manner that results in creation and propagation (in both afferent and efferent directions) of action potentials within neuronal fibers comprising a cervical vagus nerve of a patient. The patient's physiology is periodically monitored via a physiological sensor included in the implantable neurostimulator. Upon sensing a condition indicative of tachyarrhythmia, therapy delivery is switched to delivering the restorative dose to the vagus nerve. Upon sensing a condition indicative of a recurrence of the tachyarrhythmia, the therapeutic delivery of the restorative dose to the vagus nerve is continued via the pulse generator.

By improving autonomic balance and cardiovascular regulatory function, therapeutic VNS operates acutely to decrease heart rate, reflexively increase heart rate variability and coronary flow, reduce cardiac workload through vasodilation, and improve left ventricular relaxation without aggravating comorbid tachyarrhythmia or other cardiac arrhythmic conditions. Over the long term, low dosage VNS provides the chronic benefits of decreased negative cytokine production, increased baroreflex sensitivity, increased respiratory gas exchange efficiency, favorable gene expression, renin-angiotensin-aldosterone system down-regulation, and anti-arrhythmic, anti-apoptotic, and ectopy-reducing anti-inflammatory effects.

Still other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein are described embodiments by way of illustrating the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments and its several details are capable of modifications in various obvious respects, all without departing from the spirit and the scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front anatomical diagram showing, by way of example, placement of an implantable vagus stimulation device in a male patient, in accordance with one embodiment.

FIGS. 2A and 2B are diagrams respectively showing the implantable neurostimulator and the simulation therapy lead of FIG. 1.

FIG. 3 is a graph showing, by way of example, the relationship between the targeted therapeutic efficacy and the extent of potential side effects resulting from use of the implantable neurostimulator of FIG. 1.

FIG. 4 is a graph showing, by way of example, the optimal duty cycle range based on the intersection depicted in FIG. 3.

FIG. 5 is a timing diagram showing, by way of example, a stimulation cycle and an inhibition cycle of VNS as provided by implantable neurostimulator of FIG. 1.

FIG. 6 is a flow diagram showing an implantable neurostimulator-implemented method for managing tachyarrhythmias through vagus nerve stimulation, in accordance with one embodiment.

FIG. 7 is a flow diagram showing a routine for storing operating modes for use with the method of FIG. 6.

FIG. 8 is a flow diagram showing an optional routine for determining an onset or presence of arrhythmia with a recency filter for use with the method of FIG. 6.

DETAILED DESCRIPTION

Changes in autonomic control of the cardiovascular systems of patients suffering from CHF and other cardiovascular diseases push the autonomic nervous system out of balance and favor increased sympathetic and decreased parasympathetic central outflow. The imbalance is accompanied by pronounced elevation of basal heart rate arising from chronic sympathetic hyperactivation along the neurocardiac axis.

Peripheral neurostimulation therapies that target the imbalance of the autonomic nervous system have been shown to improve clinical outcomes in patients treated for three to twelve months. Specifically, bi-directional autonomic regulation therapy results in simultaneous creation and propagation of efferent and afferent action potentials within afferent and efferent nerve fibers comprising the vagus nerve. The therapy directly restores autonomic balance by engaging both medullary and cardiovascular reflex control components of the autonomic nervous system. Upon stimulation of the cervical vagus nerve, action potentials propagate away from the stimulation site in two directions, efferently toward the heart and afferently toward the brain. Efferent action potentials influence the intrinsic cardiac nervous system and the heart, while afferent action potentials influence central elements of the nervous system.

An implantable vagus nerve stimulator with integrated heart rate sensor, such as used to treat drug-refractory epilepsy and depression, can be adapted for use in managing chronic cardiac dysfunction through therapeutic bi-directional vagal stimulation. In addition, an integrated heart rate sensor can provide continual leadless heart rate monitoring that can be used in detecting arrhythmia, particularly VT, and confirming therapeutic efficacy. FIG. 1 is a front anatomical diagram showing, by way of example, placement of an implantable vagus nerve stimulation (VNS) device 11 in a male patient 10, in accordance with one embodiment. The VNS provided through the stimulation device 11 operates under several mechanisms of action. These mechanisms include increasing parasympathetic outflow and inhibiting sympathetic effects by blocking norepinephrine release. More importantly, VNS triggers the release of acetylcholine (ACh) into the synaptic cleft, which has beneficial anti-arrhythmic, anti-apoptotic, and ectopy-reducing anti-inflammatory effects.

The implantable vagus stimulation device 11 includes at least three implanted components, an implantable neurostimulator 12, a therapy lead 13, and helical electrodes 14. The implantable vagus stimulation device 11 can be remotely accessed following implant through an external programmer by which the neurostimulator 12 can be remotely checked and programmed by healthcare professionals; an external magnet, such as described in commonly-assigned U.S. Patent application, entitled “ Implantable Device For Facilitating Control Of Electrical Stimulation Of Cervical Vagus Nerves For Treatment Of Chronic Cardiac Dysfunction,” Ser. No. 13/314,130, filed on Dec. 7, 2011, now U.S. Pat. No. 8,600,505, issued Dec. 3, 2013, the disclosure of which is incorporated by reference, for basic patient control; and an electromagnetic controller, such as described in commonly-assigned U.S. Patent application, entitled “Vagus Nerve Neurostimulator With Multiple Patient-Selectable Modes For Treating Chronic Cardiac Dysfunction,” Ser. No. 13/352,244, filed on Jan. 17, 2012, now U.S. Pat. No. 8,571,654, issued Oct. 29, 2013, the disclosure of which is incorporated by reference, that enables the patient 10 to exercise increased control over therapy delivery and suspension. Together, the implantable vagus stimulation device 11 and one or more of the external components form a VNS therapeutic delivery system.

The neurostimulator 12 is implanted in the patient's right or left pectoral region generally on the same side (ipsilateral) as the vagus nerve 15, 16 to be stimulated, although other neurostimulator-vagus nerve configurations, including contra-lateral and bi-lateral are possible. The helical electrodes 14 are generally implanted on the vagus nerve 15, 16 about halfway between the clavicle 19 a-b and the mastoid process. The therapy lead 13 and helical electrodes 14 are implanted by first exposing the carotid sheath and chosen vagus nerve 15, 16 through a latero-cervical incision on the ipsilateral side of the patient's neck 18. The helical electrodes 14 are then placed onto the exposed nerve sheath and tethered. A subcutaneous tunnel is formed between the respective implantation sites of the neurostimulator 12 and helical electrodes 14, through which the therapy lead 13 is guided to the neurostimulator 12 and securely connected.

In one embodiment, the neural stimulation is provided in a low level maintenance dose independent of cardiac cycle and, upon sensing a condition indicative of spontaneous tachyarrhythmia, the stimulation device 11 delivers a restorative dose of VNS at a higher level of intensity than the maintenance dose. VNS maintenance dose delivery is only continually resumed if, during post-suspension monitoring, tachyarrhythmia is not found to recur. The stimulation device 11 bi-directionally stimulates the vagus nerve 15, 16 through multimodal application of continuously-cycling, intermittent and periodic electrical stimuli, which are parametrically defined through stored stimulation parameters and timing cycles, as further described infra with reference to FIG. 7. In a further embodiment, bradycardia in VNS-titrated patients can be managed through suspension of on-going low-level VNS, such as described in commonly-assigned U.S. Patent application, entitled “ Implantable Neurostimulator-Implemented Method for Managing Bradycardia through Vagus Nerve Stimulation,” Ser. No. 13/554,656, filed on Jul. 20, 2012, pending, the disclosure of which is incorporated by reference. In a still further embodiment, prolonged activation of the sympathetic nervous system during post-exercise recovery periods, particularly in patients with CCD, can be managed through application of an enhanced dose of VNS, such as described in commonly-assigned U.S. Patent application, entitled “Implantable Neurostimulator-Implemented Method for Enhancing Post-Exercise Recovery through Vagus Nerve Stimulation,” Ser. No. 13/673,795, filed on Nov. 9, 2012 pending, the disclosure of which is incorporated by reference.

Both sympathetic and parasympathetic nerve fibers are stimulated. Cervical vagus nerve stimulation results in propagation of action potentials from the site of stimulation in a manner that results in creation and propagation (in both afferent and efferent directions) of action potentials within neuronal fibers comprising the cervical vagus nerve to restore cardiac autonomic balance. Afferent action potentials propagate toward the parasympathetic nervous system's origin in the medulla in the nucleus ambiguus, nucleus tractus solitarius, and the dorsal motor nucleus, as well as toward the sympathetic nervous system's origin in the intermediolateral cell column of the spinal cord. Efferent action potentials propagate toward the heart 17 to activate the components of the heart's intrinsic nervous system. Either the left or right vagus nerve 15, 16 can be stimulated by the stimulation device 11. The right vagus nerve 16 has a moderately lower stimulation threshold than the left vagus nerve 15 for heart rate affects at the same parametric levels.

The VNS therapy is delivered autonomously to the patient's vagus nerve 15, 16 through three implanted components that include a neurostimulator 12, therapy lead 13, and helical electrodes 14. FIGS. 2A and 2B are diagrams respectively showing the implantable neurostimulator 12 and the simulation therapy lead 13 of FIG. 1. In one embodiment, the neurostimulator 12 can be adapted from a VNS Therapy AspireSR Model 106 pulse generator, manufactured and sold by Cyberonics, Inc., Houston, Tex., although other manufactures and types of single-pin receptacle implantable VNS neurostimulators with integrated leadless heart rate sensors could also be used. The stimulation therapy lead 13 and helical electrodes 14 are generally fabricated as a combined assembly and can be adapted from a Model 302 lead, PerenniaDURA Model 303 lead, or PerenniaFLEX Model 304 lead, also manufactured and sold by Cyberonics, Inc., in two sizes based on helical electrode inner diameter, although other manufactures and types of single-pin receptacle-compatible therapy leads and electrodes could also be used.

Referring first to FIG. 2A, the neurostimulator 12 provides multimodal vagal stimulation. In a maintenance mode, continuously-cycling, intermittent and periodic ON-OFF cycles of VNS are delivered that produce action potentials in the underlying nerves that propagate bi-directionally. In a restorative mode, VNS tuned to prevent initiation of or disrupt tachyarrhythmia through continuously-cycling, intermittent and periodic ON-OFF cycles of VNS delivered at higher intensity, which could be higher output current, higher duty cycle, higher frequency, longer pulse width, or a combination of the foregoing parameters, than the maintenance dose are delivered in response to the onset or progression of tachyarrhythmias.

The neurostimulator 12 includes an electrical pulse generator that is tuned to restore autonomic balance by triggering action potentials that propagate both afferently and efferently within the vagus nerve 15, 16. The neurostimulator 12 is enclosed in a hermetically sealed housing 21 constructed of a biocompatible, implantation-safe material, such as titanium. The housing 21 contains electronic circuitry 22 powered by a primary battery 23, such as a lithium carbon monoflouride battery. The electronic circuitry 22 is implemented using complementary metal oxide semiconductor integrated circuits that include a microprocessor controller that executes a control program according to stored stimulation parameters and timing cycles; a voltage regulator that regulates system power; logic and control circuitry, including a recordable memory 29 within which the stimulation parameters are stored, that controls overall pulse generator function, receives and implements programming commands from the external programmer, or other external source, collects and stores telemetry information, processes sensory input, and controls scheduled and sensory-based therapy outputs; a transceiver that remotely communicates with the external programmer using radio frequency signals; an antenna, which receives programming instructions and transmits the telemetry information to the external programmer; and a reed switch 30 that provides remote access to the operation of the neurostimulator 12 using an external programmer, a simple patient magnet, or an electromagnetic controller. The recordable memory 29 can include both volatile (dynamic) and persistent (static) forms of memory, such as firmware within which the stimulation parameters and timing cycles can be stored. Other electronic circuitry and components are possible.

Externally, the neurostimulator 12 includes a header 24 to securely receive and connect to the therapy lead 13. In one embodiment, the header 24 encloses a receptacle 25 into which a single pin for the therapy lead 13 can be received, although two or more receptacles could also be provided, along with the requisite additional electronic circuitry 22. The header 24 internally includes a lead connector block (not shown) and a set of set screws 26.

The housing 21 also contains a heart rate sensor 31 that is electrically interfaced with the logic and control circuitry, which receives the patient's sensed heart rate as sensory inputs. The heart rate sensor 31 monitors heart rate using an ECG-type electrode. Through the electrode, the patient's heart beat can be sensed by detecting ventricular depolarization. In a further embodiment, a plurality of electrodes can be used to sense voltage differentials between electrode pairs, which can undergo signal processing for cardiac physiological measures, for instance, detection of the P-wave, QRS complex, and T-wave. The heart rate sensor 31 provides the sensed heart rate to the control and logic circuitry as sensory inputs that can be used to determine the onset or presence of arrhythmias, particularly VT, as further described infra with reference to FIG. 8.

The neurostimulator 12 is preferably interrogated prior to implantation and throughout the therapeutic period with a healthcare provider-operable external programmer and programming wand (not shown) for checking proper operation, downloading recorded data, diagnosing problems, and programming operational parameters. Generally, use of the external programmer is restricted to healthcare providers, while more limited manual control is provided to the patient through “magnet mode.” In one embodiment, the external programmer executes application software specially designed to interrogate the neurostimulator 12. The programming computer interfaces to the programming wand through a standardized wired or wireless data connection. The programming wand can be adapted from a Model 201 Programming Wand, manufactured and sold by Cyberonics, Inc. and the application software can be adapted from the Model 250 Programming Software suite, licensed by Cyberonics, Inc. Other configurations and combinations of external programmer, programming wand and application software are possible.

The neurostimulator 12 delivers VNS under control of the electronic circuitry 22. The stored stimulation parameters are programmable. Each stimulation parameter can be independently programmed to define the characteristics of the cycles of therapeutic stimulation and inhibition to ensure optimal stimulation for a patient 10. The programmable stimulation parameters include output current, signal frequency, pulse width, signal ON time, signal OFF time, magnet activation (for VNS specifically triggered by “magnet mode”), and reset parameters. Other programmable parameters are possible. In addition, sets or “profiles” of pre-selected stimulation parameters can be provided to physicians with the external programmer and fine-tuned to a patient's physiological requirements prior to being programmed into the neurostimulator 12, such as described in commonly-assigned U.S. Patent application, entitled “Computer-Implemented System and Method for Selecting Therapy Profiles of Electrical Stimulation of Cervical Vagus Nerves for Treatment of Chronic Cardiac Dysfunction,” Ser. No. 13/314,138, filed on Dec. 7, 2011, now U.S. Pat. No. 8,630,709, the disclosure of which is incorporated by reference.

Referring next to FIG. 2B, the therapy lead 13 delivers an electrical signal from the neurostimulator 12 to the vagus nerve 15, 16 via the helical electrodes 14. On a proximal end, the therapy lead 13 has a lead connector 27 that transitions an insulated electrical lead body to a metal connector pin 28. During implantation, the connector pin 28 is guided through the receptacle 25 into the header 24 and securely fastened in place using the set screws 26 to electrically couple the therapy lead 13 to the neurostimulator 12. On a distal end, the therapy lead 13 terminates with the helical electrode 14, which bifurcates into a pair of anodic and cathodic electrodes 62 (as further described infra with reference to FIG. 4). In one embodiment, the lead connector 27 is manufactured using silicone and the connector pin 28 is made of stainless steel, although other suitable materials could be used, as well. The insulated lead body 13 utilizes a silicone-insulated alloy conductor material.

Preferably, the helical electrodes 14 are placed over the cervical vagus nerve 15, 16 at the location below where the superior and inferior cardiac branches separate from the cervical vagus nerve. In alternative embodiments, the helical electrodes may be placed at a location above where one or both of the superior and inferior cardiac branches separate from the cervical vagus nerve. In one embodiment, the helical electrodes 14 are positioned around the patient's vagus nerve oriented with the end of the helical electrodes 14 facing the patient's head. In an alternate embodiment, the helical electrodes 14 are positioned around the patient's vagus nerve 15, 16 oriented with the end of the helical electrodes 14 facing the patient's heart 17. At the distal end, the insulated electrical lead body 13 is bifurcated into a pair of lead bodies that are connected to a pair of electrodes proper. The polarity of the electrodes could be configured into a monopolar cathode, a proximal anode and a distal cathode, or a proximal cathode and a distal anode.

Therapeutically, the VNS is delivered as a multimodal set of therapeutic and event-based doses, which are system output behaviors that are pre-specified within the neurostimulator 12 through the stored stimulation parameters and timing cycles implemented in firmware and executed by the microprocessor controller. The therapeutic doses include a cardiac cycle-independent maintenance dose that includes continuously-cycling, intermittent and periodic cycles of electrical stimulation during periods in which the pulse amplitude is greater than 0 mA (“therapy ON”) and during periods in which the pulse amplitude is 0 mA (“therapy OFF”). The therapeutic doses also include a restorative dose at a higher level of intensity than the maintenance dose, which could be higher output current, higher duty cycle, higher frequency, longer pulse width, or a combination of the foregoing parameters, in response to the presence of tachyarrhythmias.

The neurostimulator 12 can operate either with or without an integrated heart rate sensor (provided that patient physiology can be monitored through some other type of sensing mechanism), such as respectively described in commonly-assigned U.S. Patent application, entitled “ Implantable Device for Providing Electrical Stimulation of Cervical Vagus Nerves for Treatment of Chronic Cardiac Dysfunction with Leadless Heart Rate Monitoring,” Ser. No. 13/314,126, filed on Dec. 7, 2011, pending, and U.S. Patent application, entitled “Implantable Device for Providing Electrical Stimulation of Cervical Vagus Nerves for Treatment of Chronic Cardiac Dysfunction,” Ser. No. 13/314,119, filed on Dec. 7, 2011, pending, the disclosures of which are hereby incorporated by reference herein in their entirety. Additionally, where an integrated leadless heart rate monitor is available, the neurostimulator 12 can provide autonomic cardiovascular drive evaluation and self-controlled titration, such as respectively described in commonly-assigned U.S. Patent application, entitled “Implantable Device for Evaluating Autonomic Cardiovascular Drive in a Patient Suffering from Chronic Cardiac Dysfunction,” Ser. No. 13/314,133, filed on Dec. 7, 2011, pending, and U.S. Patent application, entitled “ Implantable Device for Providing Electrical Stimulation of Cervical Vagus Nerves for Treatment of Chronic Cardiac Dysfunction with Bounded Titration,” Ser. No. 13/314,135, filed on Dec. 7, 2011, now U.S. Pat. No. 8,577,458, issued Nov. 5, 2013, the disclosures of which are incorporated by reference. Finally, the neurostimulator 12 can be used to counter natural circadian sympathetic surge upon awakening and manage the risk of cardiac arrhythmias during or attendant to sleep, particularly sleep apneic episodes, such as respectively described in commonly-assigned U.S. Patent application, entitled “Implantable Neurostimulator-Implemented Method For Enhancing Heart Failure Patient Awakening Through Vagus Nerve Stimulation,” Ser. No. 13/673,811, filed on Nov. 9, 2012, now U.S. Pat. No. 8,923,964, issued Dec. 30, 2014, the disclosure of which is incorporated by reference.

Therapeutically, VNS is delivered as a heart failure therapy independent of cardiac cycle and in a maintenance dose having an intensity that is insufficient to elicit side-effects, such as cardiac arrhythmias. The VNS can be delivered with a periodic duty cycle in the range of 2% to 89% with a preferred range of around 4% to 36% that is delivered as a low intensity maintenance dose. The selection of duty cycle is a tradeoff among competing medical considerations. FIG. 3 is a graph 40 showing, by way of example, the relationship between the targeted therapeutic efficacy 43 and the extent of potential side effects 44 resulting from use of the implantable neurostimulator 12 of FIG. 1. The x-axis represents the duty cycle 41. The duty cycle is determined by dividing the stimulation ON time by the sum of the ON and OFF times of the neurostimulator 12 during a single ON-OFF cycle. However, the stimulation time may also need to include ramp-up time and ramp-down time, where the stimulation frequency exceeds a minimum threshold (as further described infra with reference to FIG. 5). The y-axis represents physiological response 42 to VNS therapy. The physiological response 42 can be expressed quantitatively for a given duty cycle 41 as a function of the targeted therapeutic efficacy 43 and the extent of potential side effects 44, as described infra. The maximum level of physiological response 42 (“max”) signifies the highest point of targeted therapeutic efficacy 43 or potential side effects 44.

Targeted therapeutic efficacy 43 and the extent of potential side effects 44 can be expressed as functions of duty cycle 41 and physiological response 42. The targeted therapeutic efficacy 43 represents the intended effectiveness of VNS in provoking a beneficial physiological response for a given duty cycle and can be quantified by assigning values to the various acute and chronic factors that contribute to the physiological response 42 of the patient 10 due to the delivery of therapeutic VNS. Acute factors that contribute to the targeted therapeutic efficacy 43 include beneficial changes in heart rate variability and increased coronary flow, reduction in cardiac workload through vasodilation, and improvement in left ventricular relaxation. Chronic factors that contribute to the targeted therapeutic efficacy 43 include improved cardiovascular regulatory function, as well as decreased negative cytokine production, increased baroreflex sensitivity, increased respiratory gas exchange efficiency, favorable gene expression, renin-angiotensin-aldosterone system down-regulation, anti-arrhythmic, anti-apoptotic, and ectopy-reducing anti-inflammatory effects. These contributing factors can be combined in any manner to express the relative level of targeted therapeutic efficacy 43, including weighting particular effects more heavily than others or applying statistical or numeric functions based directly on or derived from observed physiological changes. Empirically, targeted therapeutic efficacy 43 steeply increases beginning at around a 5% duty cycle, and levels off in a plateau near the maximum level of physiological response at around a 30% duty cycle. Thereafter, targeted therapeutic efficacy 43 begins decreasing at around a 50% duty cycle and continues in a plateau near a 25% physiological response through the maximum 100% duty cycle.

The intersection 45 of the targeted therapeutic efficacy 43 and the extent of potential side effects 44 represents one optimal duty cycle range for VNS. FIG. 4 is a graph 50 showing, by way of example, the optimal duty cycle range 53 based on the intersection 45 depicted in FIG. 3. The x-axis represents the duty cycle 51 as a percentage of stimulation time over inhibition time. The y-axis represents therapeutic points 52 reached in operating the neurostimulator 12 at a given duty cycle 51. The optimal duty range 53 is a function 54 of the intersection 44 of the targeted therapeutic efficacy 43 and the extent of potential side effects 44. The therapeutic operating points 52 can be expressed quantitatively for a given duty cycle 51 as a function of the values of the targeted therapeutic efficacy 43 and the extent of potential side effects 44 at their point of intersection in the graph 40 of FIG. 3. The optimal therapeutic operating point 55 (“max”) signifies a tradeoff that occurs at the point of highest targeted therapeutic efficacy 43 in light of lowest potential side effects 44 and that point will typically be found within the range of a 5% to 30% duty cycle 51. Other expressions of duty cycles and related factors are possible.

Therapeutically and in the absence of patient physiology of possible medical concern, such as cardiac arrhythmias, VNS is delivered in a low level maintenance dose that uses alternating cycles of stimuli application (ON) and stimuli inhibition (OFF) that are tuned to activate both afferent and efferent pathways. Stimulation results in parasympathetic activation and sympathetic inhibition, both through centrally-mediated pathways and through efferent activation of preganglionic neurons and local circuit neurons. FIG. 5 is a timing diagram showing, by way of example, a stimulation cycle and an inhibition cycle of VNS 60 as provided by implantable neurostimulator 12 of FIG. 1. The stimulation parameters enable the electrical stimulation pulse output by the neurostimulator 12 to be varied by both amplitude (output current 66) and duration (pulse width 64). The number of output pulses delivered per second determines the signal frequency 63. In one embodiment, a pulse width in the range of 100 to 250 μsec delivers between 0.02 and 50 mA of output current at a signal frequency of about 20 Hz, although other therapeutic values could be used as appropriate.

In the simplest case, the stimulation time is the time period during which the neurostimulator 12 is ON and delivering pulses of stimulation. The OFF time 65 is always the time period occurring in-between stimulation times 61 during which the neurostimulator 12 is OFF and inhibited from delivering stimulation. In one embodiment, the neurostimulator 12 implements a ramp-up time 67 and a ramp-down time 68 that respectively precede and follow the ON time 62 during which the neurostimulator 12 is ON and delivering pulses of stimulation at the full output current 66. The ramp-up time 67 and ramp-down time 68 are used when the stimulation frequency is at least 10 Hz, although other minimum thresholds could be used, and both ramp-up and ramp-down times 67, 68 last two seconds, although other time periods could also be used. The ramp-up time 67 and ramp-down time 68 allow the strength of the output current 66 of each output pulse to be gradually increased and decreased, thereby avoiding deleterious reflex behavior due to sudden delivery or inhibition of stimulation at a programmed intensity.

The triggering of CHF compensatory mechanisms underlying a chronic cardiac dysfunction increases the risk of tachyarrhythmias. Although delivered in a maintenance dose having an intensity that is insufficient to elicit side-effects, such as cardiac arrhythmias, therapeutic VNS can nevertheless potentially ameliorate pathological tachyarrhythmias in some patients. Although VNS has been shown to decrease defibrillation threshold, VNS will not terminate VF in the absence of defibrillation. VNS prolongs ventricular action potential duration, so may be effective in terminating VT. In addition, the effect of VNS on the AV node may be beneficial in patients with AF by slowing conduction to the ventricles and controlling ventricular rate.

Therapeutic maintenance dose VNS therapy can be suspended upon the occurrence of tachyarrhythmia and replaced with the delivery of higher intensity VNS that is tuned to prevent initiation of or disrupt tachyarrhythmia, after which therapy only resumes if tachyarrhythmia does not recur. FIG. 6 is a flow diagram showing an implantable neurostimulator-implemented method for managing tachyarrhythmias through vagus nerve stimulation 70, in accordance with one embodiment. The method is implemented on the stimulation device 11, the operation of which is parametrically defined through stored stimulation parameters and timing cycles.

Preliminarily, an implantable neurostimulator 12 with an integrated heart rate sensor 31, which includes a pulse generator 11, a nerve stimulation therapy lead 13, and a pair of helical electrodes 14, is provided (step 71). In an alternative embodiment, electrodes may be implanted with no implanted neurostimulator or leads. Power may be provided to the electrodes from an external power source and neurostimulator through wireless RF or inductive coupling. Such an embodiment may result in less surgical time and trauma to the patient. Furthermore, the integrated heart rate sensor 31 could be omitted in lieu of other types of sensing mechanisms for measuring the patient's physiology.

The pulse generator stores a set of operating modes (step 72) that parametrically defines both a low level maintenance dose and a high level restorative dose of the stimulation, as further described infra with reference to FIG. 7. Therapeutic VNS, as parametrically defined by the maintenance dose operating mode, is delivered to at least one of the vagus nerve (step 73). The pulse generator 11 delivers electrical therapeutic stimulation to the cervical vagus nerve of a patient 10 in a manner that results in creation and propagation (in both afferent and efferent directions) of action potentials within neuronal fibers of either the left or right vagus nerve 15, 16 independent of cardiac cycle.

During maintenance dose therapy delivery, the patient's normative physiology, which is physiology during normal sinus rhythm, is checked for tachyarrhythmias (step 74), as further described infra with reference to FIGS. 7 and 8. In general, the onset or presence of pathological tachyarrhythmia can be determined by heart rate or normal sinus rhythm through an endocardial electrogram, as well as rhythm stability, onset characteristics, and similar rate and rhythm morphological indicators, as conventionally used in cardiac rhythm management devices, such as described in K. Ellenbogen et al., “Clinical Cardiac Pacing and Defibrillation,” Ch. 3, pp. 68-126 (2d ed. 2000), the disclosure of which is incorporated by reference. QRS complex alone may be insufficient to identify a specific type of tachyarrhythmia. A heart rate in excess of 100 bpm, for instance, can indicate the onset of tachyarrhythmia. Notwithstanding, while a narrow QRS complex usually indicates SVT tachycardia, a wide QRS complex can indicate any form of tachycardia, including VT tachycardia, as well as SVT and sinus tachycardia, and other indicia of tachycardia or tachyarrhythmia may be necessary before concluding that a change in therapeutic maintenance dose delivery is appropriate. For example, a wide QRS complex combined with a heart rate between around 120 bpm to 250 bpm strongly indicates the presence of VT. In a further embodiment, the onset or presence of arrhythmias, particularly VT, can be determined with the assistance of a recency filter, as further described supra with reference to FIG. 8. Still other physiological measures and indications of tachyarrhythmias are possible.

If a monitored physiological condition is indicative of tachyarrhythmia, that is, the patient's physiology indicates the onset or presence of tachyarrhythmia (step 75), the delivery of the maintenance dose is suspended and replaced with the delivery of a restorative dose of higher intensity VNS that is tuned to prevent initiation of or disrupt tachyarrhythmia (step 76). The patient's physiology is periodically monitored during the delivery of the restorative dose (step 77), the delivery of which is maintained (steps 76-77) while the tachyarrhythmia condition continues (step 78). If, after multiple checks of the patient's physiology (step 77), the arrhythmia is not responding (step 80) to the delivery of the restorative dose (step 76), absent an improvement in heart rhythm, such as a decrease in the rate of the arrhythmia (step 77), restorative dose delivery is discontinued and treatment reverts to delivering the maintenance dose (step 81) after a set period of time following the termination of the arrhythmia (step 78).

In a further embodiment, delivery of the restorative dose can be manually suspended by providing the neurostimulator 12 with a magnetically-actuated reed switch that suspends delivery of the maintenance dose and resumes delivery of the restorative dose, such as when the maintenance dose is tolerable to the patient 10, while the restorative dose is intolerable.

In a still further embodiment, the intensity of the restorative dose can be increased as necessary (step 79). For non-life-threatening or non-paroxysmal tachyarrhythmias, the intensity of the restorative dose is progressively increased, while for life-threatening or paroxysmal arrhythmias, a strong restorative dose of significantly higher intensity is used right away, due to the lack of time to ramp up the intensity progressively.

The recordable memory 29 in the electronic circuitry 22 of the neurostimulator 12 (shown in FIG. 2A) stores the stimulation parameters that control the overall functionality of the pulse generator 11 in providing VNS therapy. FIG. 7 is a flow diagram showing a routine 90 for storing operating modes for use with the method 70 of FIG. 6. Two operating modes are stored, which include a maintenance dose of VNS tuned to restore cardiac autonomic balance (step 91) through continuously-cycling, intermittent and periodic electrical pulses and a restorative dose tuned to prevent initiation of or disrupt tachyarrhythmia (step 92) through periodic electrical pulses delivered at higher intensity than the maintenance dose.

In one embodiment, the autonomic regulation therapy is provided in a low level maintenance dose independent of cardiac cycle to activate both parasympathetic afferent and efferent nerve fibers in the vagus nerve simultaneously and a high level restorative dose. In the maintenance dose, a pulse width in the range of 250 to 500 μsec delivering between 0.02 and 1.0 mA of output current at a signal frequency in the range of 10 to 20 Hz, and a duty cycle of 5 to 30%, although other therapeutic values could be used as appropriate.

Different restorative doses are provided in response to different tachyarrhythmias. The restorative dose settings are physician-programmable. For a default restorative dose, the stimulation parameters would be in the same range as the maintenance dose, but would be moderately higher, with a pulse width again in the range of 250 to 500 μsec delivering between 1.5 and 2.0 mA of output current at a signal frequency in the range of 10 to 20 Hz. The duty cycle may change significantly from nominally 10% to temporarily 50% or 100%, although other therapeutic values could be used as appropriate. In addition, for non-life-threatening or non-paroxysmal tachyarrhythmias, the intensity of the restorative dose is progressively increased over time by increasing output current, duty cycle, or frequency, lengthening pulse width, or through a combination of the foregoing parameters. As well, discretely-defined restorative doses, each using different sets of parameters, may be delivered in the course of treating a single continuing tachyarrhythmic event, such as for life-threatening or paroxysmal arrhythmias that rapidly generate and require a significantly stronger restorative dose with no ramp up time.

The physiology of the patient 10 is monitored during the delivery of the maintenance dose and while undertaking rehabilitative measures, such as restorative dose delivery to counter an occurrence of arrhythmia, particularly VT, or other significant increase, decrease, or entrainment of cardiac rhythm. FIG. 8 is a flow diagram showing an optional routine 100 for determining an onset or presence of arrhythmias with a recency filter for use with the method 70 of FIG. 6. Normative physiology refers to the relative physiology of a patient suffering from CHF or other form of chronic cardiac dysfunction and the physiology of a particular CHF patient may vary from the norm as observed in a healthy non-CHF individual. Physiological thresholds of arrhythmogenesis are defined (step 101) that are used to establish conditions that indicate a departure from normal sinus rhythm. Periodically, the normative physiology of the patient 10 is recorded in the recordable memory 29 (shown in FIG. 2A) (step 102). The normative physiology can include heart rate or normal sinus rhythm, as sensed by a physiological sensor, such as a heart rate monitor 31, as well as other available physiological data, for instance, as derivable from an endocardial electrogram. In a further embodiment, statistics can be stored in the recordable memory 29 for storage efficiency, instead of the raw sensed heart rate data. For instance, a binned average heart rate could be stored as representative of the patient's overall heart rate during a fixed time period. Based on the recorded normative physiology, a statistical average can be determined (step 105). If the statistical average exceeds the arrhythmogenic threshold (step 106), arrhythmia likely is present (step 107) and modification of the operating mode of the pulse generator 11 is indicated. The arrhythmia can include various forms of tachyarrhythmia, including atrial fibrillation (AF), ventricular tachyarrhythmias (ventricular tachycardia (VT) and ventricular fibrillation (VF)), and atrial flutter, and bradyarrhythmia, including atrial bradycardia, atrioventricular bradycardia, and ventricular bradycardia.

In a further embodiment, a form of recency filter is used to statistically favor recent events. By way of example, more statistical weight can be assigned to recently recorded normative physiology (step 103) and less statistical weight can be assigned to normative physiology recorded in the past (step 104). The lower weighting of older physiology values helps form a long-term running average that can be treated as a trailing baseline. A sliding window of physiology values collected, for example, over the past week can be applied to typify current patient condition. Weighted statistical averages are determined for recent and past normative physiology (step 105) and recent (foreground) and past (background or trailing) physiology averages can be compared (step 106) to identify arrhythmogenesis or other consideration.

In a still further embodiment, the sensed heart rate data can be used to analyze therapeutic efficacy and patient condition. For instance, statistics could be determined from the sensed heart rate, either onboard by the neurostimulator 12 or by an external device, such as a programming computer following telemetric data retrieval. The sensed heart rate data statistics can include determining a minimum heart rate over a stated time period, a maximum heart rate over a stated time period, an average heart rate over a stated time period, and a variability of heart rate over a stated period, where the stated period could be a minute, hour, day, week, month, or other selected time interval. Still other uses of the heart rate sensor 31 and the sensed heart rate data are possible

In still further embodiments, the suspension and resumption of either or both the delivery of the maintenance dose and the restorative dose can be titrated to gradually withdraw or introduce their respective forms of VNS. As well, both forms of VNS therapy delivery can be manually suspended by providing the neurostimulator 12 with a magnetically-actuated reed switch that suspends delivery of the maintenance dose and the restorative dose, as applicable, in response to a remotely applied magnetic signal.

While the invention has been particularly shown and described as referenced to the embodiments thereof, those skilled in the art will understand that the foregoing and other changes in form and detail may be made therein without departing from the spirit and scope. 

What is claimed is:
 1. An implantable neurostimulator for managing tachyarrhythmias through vagus nerve stimulation, comprising: a pulse generator configured to generate electrical stimulation; a processor; and a memory having instructions stored thereon that, when executed by the processor, cause the processor to: define a maintenance dose of the electrical stimulation; define a restorative dose of the electrical stimulation comprising electrical pulses delivered at a higher intensity than the maintenance dose; deliver the maintenance dose to an electrode assembly, via the pulse generator, prior to delivering the restorative dose, prior to sensing a condition of tachyarrhythmia, and independent of cardiac cycle; monitor a patient's normative physiology to sense the condition of tachyarrhythmia via a physiological sensor; upon sensing the condition of tachyarrhythmia, switch from delivering the maintenance dose to delivering the restorative dose; and upon sensing a condition indicative of a recurrence of the tachyarrythmia, continue delivery of the restorative dose.
 2. The implantable neurostimulator of claim 1, wherein the higher intensity of the restorative dose comprises at least one of a higher output current, higher duty cycle, higher frequency, or longer pulse width than the maintenance dose.
 3. The implantable neurostimulator of claim 1, wherein the instructions further cause the processor to sense the condition indicative of tachyarrhythmia based on at least one of an abnormal sinus rhythm of the patient or a heart rate of the patient rising above a maximum threshold.
 4. The implantable neurostimulator of claim 1, wherein the instructions further cause the processor to, upon sensing a condition indicative of a recurrence of the tachyarrhythmia, continue delivery of the restorative dose at an increased intensity.
 5. The implantable neurostimulator of claim 4, wherein the instructions further cause the processor to, upon sensing the condition indicative of a recurrence of the tachyarrhythmia, continue delivery of the restorative dose at the increased intensity by: determining whether the condition of tachyarrhythmia is at least one of a life-threatening or paroxysmal arrhythmia, wherein the at least one of a life-threatening or paroxysmal arrhythmia is associated with a first increased intensity; and upon determining that the condition of tachyarrhythmia is at least one of a life-threatening or paroxysmal arrhythmia, increase the intensity of the restorative dose to the first increased intensity.
 6. The implantable neurostimulator of claim 4, wherein the instructions further cause the processor to, upon sensing the condition indicative of a recurrence of the tachyarrhythmia, continue delivery of the restorative dose at the increased intensity by: determining whether the condition of tachyarrhythmia is at least one of a life-threatening or paroxysmal arrhythmia, wherein the at one of a life-threatening or paroxysmal arrhythmia is associated with a first increased intensity; and upon determining that the condition of tachyarrhythmia is not at least one of a life-threatening or paroxysmal arrhythmia, increase the intensity of the restorative dose to a second increased intensity less than the first increased intensity.
 7. The implantable neurostimulator of claim 1, wherein the instructions further cause the processor to, after continuing delivery of the restorative dose and upon sensing another condition indicative of a recurrence of the tachyarrhythmia, terminate delivery of the restorative dose.
 8. A method for managing tachyarrhythmia through vagus nerve stimulation, comprising: defining a maintenance dose of electrical stimulation; defining a restorative dose of the electrical stimulation comprising electrical pulses delivered at a higher intensity than the maintenance dose; delivering, using a pulse generator of an implantable medical device, the maintenance dose, prior to delivering the restorative dose, prior to sensing a condition of tachyarrhythmia, and independent of cardiac cycle; monitoring, using a sensor of the implantable medical device, a patient's normative physiology to sense the condition of tachyarrhythmia; upon sensing the condition of tachyarrhythmia, switching from delivering the maintenance dose to delivering the restorative dose; and upon sensing a condition indicative of a recurrence of the tachyarrhythmia, continuing delivery of the restorative dose.
 9. The method of claim 8, wherein the higher intensity of the restorative dose comprises at least one of a higher output current, higher duty cycle, higher frequency, or longer pulse width than the maintenance dose.
 10. The method of claim 8, wherein monitoring, using the sensor, the patient's normative physiology to sense the condition of tachyarrhythmia comprises monitoring, using the sensor, the patient's normative physiology to sense the condition of tachyarrhythmia based on at least one or an abnormal sinus rhythm of the patient or a heart rate of the patient rising above a maximum threshold.
 11. The method of claim 8, wherein, upon sensing the condition indicative of a recurrence of the tachyarrhythmia, continuing the delivery of the restorative dose comprises continuing the delivery of the restorative dose at an increased intensity.
 12. The method of claim 11, wherein, upon sensing the condition indicative of a recurrence of the tachyarrhythmia, continuing the delivery of the restorative dose at the increased intensity comprises: determining whether the condition of tachyarrhythmia is at least one of a life-threatening or paroxysmal arrhythmia, wherein the at least one of the life-threatening or paroxysmal arrhythmia is associated with a first increased intensity; and upon determining that the condition of tachyarrhythmia is at least one of a life-threatening or paroxysmal arrhythmia, increasing the intensity of the restorative dose to the first increased intensity.
 13. The method of claim 11, wherein, upon sensing the condition indicative of a recurrence of the tachyarrhythmia, continuing the delivery of the restorative dose at the increased intensity comprises: determining whether the condition of tachyarrhythmia is at least one of a life-threatening or paroxysmal arrhythmia, wherein the at least one of the life-threatening or paroxysmal arrhythmia is associated with a first increased intensity; and upon determining that the condition of tachyarrhythmia is not at least one of a life-threatening or paroxysmal arrhythmia, increasing the intensity of the restorative dose to a second increased intensity less than the first increased intensity.
 14. The method of claim 8, further comprising, after continuing delivery of the restorative dose and upon sensing another condition indicative of a recurrence of the tachyarrhythmia, terminating delivery of the restorative dose.
 15. A non-transitory computer-readable medium comprising instructions executable by a processor of an implantable medical device to perform operations comprising: defining a maintenance dose of electrical stimulation; defining a restorative dose of the electrical stimulation comprising electrical pulses delivered at a higher intensity than the maintenance dose; delivering, using a pulse generator of the implantable medical device, the maintenance dose, prior to delivering the restorative dose, prior to sensing a condition of tachyarrhythmia, and independent of cardiac cycle; monitoring, using a sensor of the implantable medical device, a patient's normative physiology to sense the condition of tachyarrhythmia; upon sensing the condition of tachyarrhythmia, switching from delivering the maintenance dose to delivering the restorative dose; and upon sensing a condition indicative of a recurrence of the tachyarrhythmia, continuing delivery of the restorative dose.
 16. The non-transitory computer-readable medium of claim 15, the higher intensity of the restorative dose comprises at least one of a higher output current, a higher duty cycle, higher frequency, or longer pulse width than the maintenance dose.
 17. The non-transitory computer-readable medium of claim 15, wherein monitoring, using the sensor, the patient' normative physiology to sense the condition of tachyarrhythmia comprises monitoring, using the sensor, the patient's normative physiology to sense the condition of tachyarrhythmia based on at least one of an abnormal sinus rhythm of the patient or a heart rate of the patient rising above a maximum threshold.
 18. The non-transitory computer-readable medium of claim 15, wherein, upon sensing the condition indicative of a recurrence of the tachyarrhythmia, continuing the delivery of the restorative dose comprises continuing the delivery of the restorative dose at an increased intensity.
 19. The non-transitory computer-readable medium of claim 18, wherein, upon sensing the condition indicative of a recurrence of the tachyarrhythmia, continuing the delivery of the restorative dose at the increased intensity comprises: determining whether the condition of tachyarrhythmia is at least one of a life-threatening or paroxysmal arrhythmia, wherein the at least one of the life-threatening or paroxysmal arrhythmia is associated with a first increased intensity; and upon determining that the condition of tachyarrhythmia is not at least one of a life-threatening or paroxysmal arrhythmia, increasing the intensity of the restorative dose to a second increased intensity less than the first increased intensity.
 20. The non-transitory computer-readable medium of claim 15, wherein the operations further comprise, after continuing delivery of the restorative dose and upon sensing another condition indicative of a recurrence of the tachyarrhythmia, terminating delivery of the restorative dose. 